The present invention relates to dual energy detectors that may find use in x-ray imaging equipment producing images which distinguish between multiple basis materials. Such systems find use in medical systems, for example, bone densitometers, and in industrial inspection systems such as airport baggage scanners.
The measurement of x-ray energy attenuated by an imaged object in two distinct energy bands can be used to determine information about the material out of which the imaged object is composed. Generally attenuation is a function of x-ray energy according to two attenuation mechanisms of photoelectric absorption and Compton scattering. These two mechanisms differ among materials of different atomic numbers. For this reason, measurements at two energies can be used to distinguish between two different basis materials.
Dual energy x-ray techniques can be used to separate bony tissue from soft tissue in medical imaging, for example, or to identify hazardous materials, for example, in baggage scanning.
In a switched mode system dual energy x-ray system, the voltage on the x-ray tube is periodically changed from a high to low voltage shifting the energy spectrum of the produced x-ray beam. A single set of solid state detectors illuminated by the beam collects measurements of the imaged object at the different energy outputs. Here x-ray detectors having broad-band sensitivity to different energies are used. A so-called K-edge filter may be placed in the beam before the x-ray detector to sharpen the output spectrum of the x-rays at the different energies.
In an alternative approach, a broad band x-ray beam may be produced having multiple energies, and the discrimination between x-ray energies may be done by the detector which provides separate output signals for received high and low x-ray energies. One type of detector used in this approach is a stacked array detector in which two detector elements are stacked on top of each other. Typically, a front most detector will measure total x-ray flux and a rearward detector will measure only higher energy x-ray photons not stopped by an intervening filter. Low energy photons may be deduced from these two measurements.
An alternative side-by-side detector construction described in the predecessor applications to this application, two rows of detector elements are placed side-by-side and scanned along the imaged object in a direction perpendicular to the rows. The detector elements of the first row has a different energy sensitivity from that of the second row. Scanning the imaged object causes the two rows to pass over the same areas of the imaged object, each making measurements. High and low energy measurements for a given location in the imaged object are made at slightly different times.
While this detector construction has a number of advantages, the fact that the high and low energy readings are made at different times may produce minor image artifacts if the patient moves between the measurements, even though the time between measurements is slight. Movements may include involuntary muscle movement in the case of a patient and/or vibrations of elastic elements in case of non-living objects.
Interpolation between multiple high or low energy measurements taken at different times to produce a virtual measurement of the high and low energies at a single, common time and single, common location is not possible with this detector design. This is because any two measurements, of either high or low energy that might be used for interpolation to a common time and common location, are displaced from that time and location by different amounts preventing the use of a common interpolation factor. In short, the architecture of the detector prevents interpolation from being used to eliminate motion artifacts.